Posted on November 25, 2009 by Scott Weese

Bug of the Month: Rotavirus

Rotavirus is an important cause of diarrhea in young horses. (It's also a major cause of diarrhea in infants, but a different rotavirus is involved). Exposure to rotavirus is very common - most, if not all, horses are exposed to it early in life. Disease only occurs in foals, but not every foal that gets exposed becomes sick. Most often, rotavirus causes diarrhea if foals between 1 and 6 months of age, with most cases occurring between 1 and 3 months of age. Foals less than 1 month of age can be affected, but it's less common.

Foals become infected by swallowing the virus, which they usually pick up from the feces of other horses (including their mares) in their environment. The exact source of infection is rarely identified. It is likely that some healthy adult horses are the reservoirs and shed the virus in their manure.

Like other types of diarrhea, rotaviral diarrhea in foals can range from mild diarrhea alone to severe diarrhea with weakness, loss of appetite and dehydration. Colic can occur because of intestinal cramping associated with diarrhea. You cannot differentiate diarrhea due to rotavirus from other types of diarrhea by appearance alone. The diarrhea must be tested to identify the cause.

There is no specific treatment for rotavirus infection, but "supportive care" is often provided. One of the primary components of supportive care is fluid therapy, which may be needed for foals that start to become weak and dehydrated. Aggressive intravenous fluid therapy may be needed in some cases, depending on the severity.

Rotavirus vaccines are available in some regions, although there is limited evidence that they have much of an effect. Few people recommend vaccination. Rather, good attention to foal management practices, hygiene and early diagnosis of infected foals is the key. Foaling mares outside on pasture has been an effective practice in some outbreaks, presumably because there is less rotavirus contamination on pasture.

Image: Colourized TEM of rotavirus particles (source: CDC PHIL 173)

Tags: , , , ,
Posted on November 13, 2009 by Maureen Anderson

Infectious Agents In Foal Diarrhea

A new study was recently published in the Journal of Veterinary Internal Medicine (Frederick et al. 2009) which looked at infectious agents found in the feces of foals with diarrhea. Specifically, they looked for rotavirus, Clostridium perfringens, C. difficile, Salmonella, parasite eggs and Cryptosporidium oocysts.

They found at least one infectious agent in the feces of 122 (55%) of the 233 foals in the study. That means, despite testing for a wide range of pathogens, they could not identify an infectious agent in 45% of the diarrheic foals. This is very similar to the situation typically found in adult horses with diarrhea. This could have happened for a number of reasons:

  1. No test is perfect. It’s possible that in some of the cases one of the test results was a “false negative,” meaning it did not detect the infectious agent even though it was there.
  2. A few cases may have been caused by other infectious agents that were not included in the diagnostic panel.  For example, Rhodococcus equi is a common cause of respiratory disease in foals, but it has also been associated with diarrhea in some cases.
  3. The diarrhea was caused by an agent of which we are unaware, and for which we have no test. Researchers are constantly looking for other bacteria, viruses or parasites that may be capable of causing or contributing to diarrhea in foals and adult horses.
  4. The diarrhea was not caused by an infectious agent. For example, the authors failed to discuss foal heat diarrhea as a cause of clinical diarrhea in very young foals. This is a well recognized cause of foal diarrhea, but no infectious agents are involved.

The most commonly identified pathogens were rotavirus (20% of cases), Clostridium perfringens (18%), Salmonella (12%) and C. difficile (5%). Overall 191 (87%%) of the foals survived, and survival was not associated with any pathogen identified in the feces (i.e. in this study, foals were not more likely to die if they had one particular pathogen in their feces than another). This must be interpreted very cautiously, however, because the study does not account for other kinds of illness in these foals, or even whether diarrhea was the primary problem for which they were referred to the hospital. Diarrhea, especially in very young foals, can be very serious because they can dehydrate very quickly and are very susceptible to shock of various kinds.

There were a few other issues with this study that are important to keep in mind as well:

  1. The group of foals they looked at ranged in age from newborns to 10-months old. The digestive tract of a foal undergoes drastic changes in the first year of life, and it is well known that certain infectious agents only cause disease in foals of particular ages. For example, different parasites may take weeks to months to develop within the intestine of a foal, so even if a foal is infected as soon as it’s born, these parasites cannot cause disease for quite some time. For this reason, it would have perhaps been more useful to look at the data separately for different age groups.
  2. There was no control group in this study. Most of the time, if an animal has diarrhea and the test you perform tells you there is a known diarrhea-causing pathogen in the feces , you assume the diarrhea is due to that agent. This is not necessarily always the case. Some pathogens are carried around by totally normal animals, who may get diarrhea for a completely different reason. So what we really need to know now is: if the authors tested 233 foals with normal feces (and the same ages), how common would each of these pathogens be?

While the data may be interesting to look at, this paper doesn't really tell us anything new that will change the way we treat or manage foals with diarrhea in general.  Nonetheless, the information may still be useful for helping to design and interpret future research studies about these pathogens and diarrheal disease in foals.

Image source: www.bbc.co.uk (credit: George Ring)

Tags: , , , , , ,